(Reuters) - Civilians including women and children will need to be dug from bunkers under a steel works that is the last holdout of resistance in Ukraine's Mariupol, President Volodymyr Zelenskiy said on Thursday, as Russian forces pressed their assault.

After failing to capture the capital Kyiv in the early weeks of a war that has killed thousands and flattened cities, towns and villages, Russia has accelerated its attacks across southern and eastern Ukraine.

Russia's military said it would pause military activity at the Azovstal steel works in the port city of Mariupol during Thursday daytime and the following two days to allow civilians to leave.

In an early morning address, Zelenskiy said Ukraine stood ready to ensure a ceasefire in Mariupol.

"It will take time simply to lift people out of those basements, out of those underground shelters. In the present conditions, we cannot use heavy equipment to clear the rubble away. It all has to be done by hand," Zelenskiy said.

Ukrainian fighters inside Azovstal are fighting "difficult, bloody battles" against Russian troops, Denis Prokopenko, a commander with Ukraine's Azov regiment, said late on Wednesday.

Ukraine's military general staff said Russia's assault on the sprawling Soviet-era plant included air support.

Moscow declared victory in Mariupol on April 21 after weeks of siege and shelling, but resistance by Ukrainian forces in Azovstal has prevented Russia completely overrunning the city.

Mariupol is a major target in Russia's efforts to cut Ukraine off from the Black Sea - vital for its grain and metals exports - and link Russian-controlled territory in the east of the country to Crimea, seized by Moscow in 2014.